1. Field of the Invention
The present invention concerns a method to ensure the removability and/or the mobility of at least one element cast in a casting compound in a module of a medical apparatus, in particular a gradient coil of a magnetic resonance apparatus.
2. Description of the Prior Art
In many medical apparatuses (such as magnetic resonance examination scanners) specific components or modules are encapsulated (cast) or impregnated with an epoxy-based resin in order to achieve electrical insulation and mechanical stabilization. Furthermore, the encapsulation allows heat to be dissipated and ensures a greater protection against environmental influences.
Examples of such encapsulated components are the gradient coils in magnetic resonance scanners. These gradient coils are essentially three-axis magnetic field coils that are encapsulated in a resin matrix (which is most often a matrix made from epoxy resin) in order to generate or to ensure the desired mechanical and electrical properties of these coils. For example, in the case of cylindrical gradient coils, the construction of the coil arrangement ensues on a likewise cylindrical metallic mandrel.
Furthermore, for shaping of an external contour of a medical apparatus or to enable through-holes and openings, it is typical to cast metallic shaped parts that are subsequently ejected from the casting compound after curing.
To enable or to ensure the removal of these shaped parts or of the mandrel for the coil construction and similar elements, care must be taken that these elements do not permanently adhere with the casting compound surrounding them.
In addition, there are modules for which a certain mobility must be ensured such as, for example, mounting devices for insert gradient coils in magnetic resonance devices. Roller bodies whose rotation capability must be ensured in spite of an encapsulation are used for the installation or altering for the position of such coils.
Such casting parts (such as, for example, the cylindrical mandrel for the coil construction) have conventionally been treated with a separation means (for example made of silicon) in order to avoid an adhesion to the surrounding casting compound. It is additionally possible to wrap the corresponding elements that are to be cast in the casting compound with a separator foil that is subsequently closely masked.
What are known as draft bevels or chamfers are additionally provided at the cast elements that, however, lead to the situation that a deviation from the ideal geometric shape that must be accepted.
For example, this means that a cylindrical mandrel for a cylindrical coil design can deviate from the ideal cylindrical shape by a few millimeters given a length of approximately 1.50 m.
Profile rods made from aluminum with an approximately rectangular cross-section are integrated into the coil construction in a similar manner and are ejected again after the curing of the casting compound. In this case as well the aluminum surface is treated with a silicone separation means and draft bevels are additionally provided, wherein the cited problems with regard to a deviation from a desired, exact shape result.
A method as well as a device for production of an actively shielded gradient coil arrangement for a magnetic resonance apparatus are known from DE 197 22 211 (A1), in which (among other things) mold cores are mounted that are removed again after a casting and curing of a casting material. These mold cores can likewise comprise aluminum and are if applicable wetted with a separation means such as silicone oil so that they can be easily removed after curing of the cast.
DD 86 489 concerns a mold core for production of hollow bodies or shaped parts made from reinforced, curable plastic resins, wherein mold cores treated with meltable separation means as already known are described. Such mold cores enable a removal from the mold with a comparably small force expenditure after the melting out of the separation means, but require a protracted and elaborate separation means treatment.